Hip-joint replacement is a widely used orthopedic procedure to correct various types of hip joint injury, such as those caused by osteoarthritis, fracture dislocations, rheumatoid arthritis, and aseptic bone necrosis. A femoral-side hip-joint prosthesis typically includes, in basic design, an elongate stem which is adapted for receipt in a cavity formed in the proximal region of the femur, and a ball-like hip-joint member carried at the upper end of the stem. To attach the prosthesis to the femur surgically, the head of the femur is removed, and the internal bone region below the cut is removed to form the cavity which will receive the prosthesis stem. The stem may be anchored in the cavity by a cementitious material such as polymethylmethacrylate (PMMA), which acts as a grout material, by bonding directly to bone, such as with hydroxyapatite and/or by biological fixation, such as by bone growth into a porous surface of the stem.
Heretofore, femoral-side hip-joint replacement has not been entirely satisfactory, particularly in active younger patients, where it is important to form a stable, long-lasting prosthetic attachment. The stem size and shape in prior art hip-joint prostheses has generally required removal of a substantial amount of hard outer cortical bone in forming a cavity which can accommodate the stem. This removal alters the load distribution within the bone, which can lead to detrimental long-term bone remodeling. Also, in the surgical technique required in prior art prostheses, the piriformis fossa is substantially reamed, and this compromises the blood supply to the proximal femur. Also, the natural iternal support canal which is formed by dense cancellous bone is removed in order to insert prior art prostheses.
A second limitation is related to the manner in which prior-art stems are anchored in the cavity. In order to permit stem receipt in the cavity, a large amount of proximal femur must be removed. This requires a relatively large prosthesis cross-section to achieve good bone/prosthesis apposition.
Attempts to anchor a prosthesis stem by press fit in a cavity, typically using a three-point fixation system have often failed to produce stable fixation, due to stem loosening because of torsion or breakdown of the three point support system.